Aeration, the process of purposely exchanging gases between the atmosphere and water, is a required aspect of many different biological, engineering and social systems. Most commonly, aeration is required to either add a gas to water when its absence compromises a desired outcome or removing a gas from water when its surplus is similarly unwanted. In both cases, engineers have created a wide variety of technologies to accomplish these tasks. Most involve physical manipulations of the physics of air-water gas exchange to enhance the exchange rate as well as clever innovations to increase the effectiveness or reduce the costs. Some of these approaches also directly influence the need for aeration.
Although the dynamics of many different gases can require enhanced rates of air-water gas exchange, one common application is the requirement for oxygen in waters that must support life. Photosynthesis, the basis of nearly all life on earth, uses sunlight energy to create the chemical bonds in organic material. Whether on land or in water, photosynthetic organisms transform carbon dioxide into organic material and use water as a source of electrons, thus making gaseous oxygen as a by-product of the water-splitting reaction. Animals and bacteria, commonly called heterotrophic organisms, use this organic material as a source of energy for their metabolism. In the process, they excrete carbon dioxide. They also must find a terminal electron acceptor for the electron transport chain in respiration. Many organisms use oxygen for this purpose, thereby re-creating pure water, while some micro-organisms can use a variety of metals as the terminal electron acceptor.
The balance of oxygen production and oxygen respiration in any ecosystem determines the net changes in the constituents that are involved in the processes. Excess photosynthesis over respiration can lead to a buildup of organic material and oxygen and a decline in carbon dioxide and nutrients. Systems that have inputs of large amounts of exogenous organic material can support the growth of stable populations of organisms, but the oxygen content, or that of any other electron acceptors, will steadily decline. The state of any system will be a function of these kinds of balances over any specific time and space scale.
For example, sewage treatment plants bring in large amounts of organic waste from urban environments with water as the carrier and break that waste down to inorganic nutrients as part of making the water safer for discharge back into the environment. This is usually done by growing micro-organisms on the organic waste to reduce the “Biological Oxygen Demand” (“BOD”). BOD is a short-hand measure for how much oxygen would be required to allow these organisms to aerobically metabolize all of the organic wastes to inorganic nutrients. Since these systems have an exogenous supply of organics, they generally require an exogenous source of oxygen. Typically, this is supplied by adding oxygen directly to the water carrying the organic waste, either through enhanced gas exchange with the oxygen in the atmosphere or through direct injection of pure oxygen.
Other managed water bodies have a related dynamic. Fish farms have a large organic loading through the feed that is added to the ponds to support the growth and metabolism of the fish. Both unused feed and fish wastes stimulate the growth of bacteria and other micro-organisms. Similarly, many man-made bodies of water have large organic loadings, including the ponds at golf-courses, small fishing lakes near farms, harbors, waste ponds near agricultural food processing plants and many others. All of these have a similar issue. The pond may become anoxic if the oxygen drops too low, which may bring negative consequences. In the fish ponds or lakes, the fish and other large animals will die. In some of these, the anoxic ponds release noxious and foul smelling gases that make them an eyesore and nuisance.
Natural bodies of water also show similar dynamics. When organic loadings are high, the scenarios are similar to the fish ponds. However, natural bodies also can have unfortunate responses to the addition of inorganic nutrients from either natural sources, for example upwelling, or manmade, for example nutrient discharge. In these cases, large populations of plants and algae will grow in a process called eutrophication. As these populations of plants use up the nutrients, the organic biomass sinks out and is subsequently consumed. While the plants are growing near the surface, they make extra oxygen which outgases to the atmosphere. As the organic material decays, it consumes oxygen until the dissolved oxygen is gone and the system becomes anaerobic. This can lead to dead-zones and other harmful ecosystem effects. These also tend to occur on spatial scales that are much too large for an effective engineering response after the organic material is present. Sometimes humans can influence the original source of inorganic nutrients, such as removing phosphates from detergents or reducing nutrient runoff from farms. However, sometimes eutrophication cannot be avoided.
While these examples are primarily centered on oxygen balance issues, there are other instances when enhanced gas exchange can be required. Large amounts of respiration will raise the amount of carbon dioxide in the water and its removal can be advantageous in some cases. Conversely, growing algae or other aquatic plants actually requires carbon dioxide as a nutrient and it must be added if the rate of growth exceeds the ability of the ecosystem to supply it naturally.
In most cases where there is too little oxygen, or too little or too much of any other gas, it is because the internal dynamics of the water exceed the natural rate of gas exchange between water and air. The typical response is to increase the rate of gas exchange through one of a variety of methods. Gas exchange rate is governed by a complex set of physical and chemical processes that are fairly well known. In general, the rate is a function of the surface area of the air-water interface, the concentration difference of the gases at the surface as indicated by their partial pressures and the mixing of water and air away from this surface to homogenize with the concentrations in the wider body of water.
Enhancements to gas exchange rate generally involve a wide variety of technologies and engineered solutions that increase the concentration gradient, increase the surface area and increase the turbulent mixing away from the interface. The simplest of these involve some combination of splashing and bubbling. Splashing puts drops of water into the air, increasing the effective amount of surface area and mixing the drops with the wider body of water on impact. Bubbling puts small bubbles in the water, again increasing surface area and, since bubbles rise, aiding in mixing. In both cases, increasing the gradient can be done either by careful choice of timing and location for the splashing/bubbling to ensure that the concentrations in the water and air are as different as possible, for example drawing water from the bottom of a pond or at night when the oxygen is lowest, or by using gas mixtures that have a higher content of the gas of interest, for example pure oxygen or CO2.
These traditional aeration techniques are generally effective. However, they are also quite expensive in terms of their energy requirements. Physical methods generally require a lot of energy to move water and air. This energy is typically supplied by electricity produced by the combustion of fossil fuels, whose volatile prices are ever increasing. Making pure gas mixtures is even more expensive. Thus, as fossil fuel and energy prices rise and as the world discusses the consequences of the emission of fossil fuel carbon dioxide, the price component of aeration begins to take a significant role in understanding the sustainability of various human practices. Technologies that can reduce these costs should have a positive impact on many human activities and businesses at the same time as they have a positive effect on the planet.